The invention relates to a luminescent alumino-silicate and/or alumino-borate glass comprising lanthanum and/or gadolinium and activated by a rare earth metal. The invention also relates to a luminescent screen provided with such a glass.
Luminescent glasses of the type described above are known from U.S. Pat. No. 4,102,805, describing luminescent alumino-silicate glasses activated by bivalent europium and also comprising a further oxide, for example, lanthanum oxide. The Japanese patent application No. 74-99610, laid open to public inspection, describes luminescent silicate and/or borate glasses activated by trivalent terbium and trivalent europium, which comprise one or more of the oxides of aluminium, lanthanum and the alkaline earth metals and furthermore an alkali metal oxide.
A drawback of the known luminescent glasses is that they generally have a low efficiency. Particularly the quantum efficiency upon excitation by ultraviolet radiation is often found to be a factor of 5 to 10 smaller than the quantum efficiencies which can be achieved with efficient crystalline materials. A further drawback of the alkali-metal containing luminescent glasses is that they generally have a large decline in the luminous flux in practical uses, for example, in discharge lamps.
Luminescent alumino-borate and/or alumino-silicate glasses activated by Tb.sup.3+ and/or Ce.sup.3+ and comprising one or more alkaline earth metal oxides are known from Netherlands patent application No. 58501107 (PHN 11.363).
It is an object of the invention to provide novel luminescent glasses having high conversion efficiencies and notably high quantum efficiencies upon excitation by ultraviolet radiation.